Merge branch 'backfill-multiple-desired-sharding-key-small-table-snippet_user_mentions' into 'master' 

Add & backfill sharding keys for snippet_user_mentions

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/180836



Merged-by: Omar Qunsul <oqunsul@gitlab.com>
Approved-by: Omar Qunsul <oqunsul@gitlab.com>
Co-authored-by: Shubham Kumar <shukumar@gitlab.com>